A Long Term Evolution (LTE) system is based on Hybrid Automatic Repeat reQuest (HARQ) for data transmission. More specifically, based on data reception status, the data receiver in an LTE system will send receiving status feedback information, i.e., an ACKnowledgement (ACK) or a Negative ACKnowledgement (NACK). Scheduling information of dynamic downlink data is transmitted through a Physical Downlink Control CHannel (PDCCH), while for a Semi-Persistent Scheduling (SPS) service, because the scheduling information is sent through the PDCCH when the downlink data is retransmitted, it is not necessary to send PDCCH information in initial transmission scheduling information of the downlink data.
In an LTE Time Division Duplex (TDD) system, Downlink Assigning Indication (DAI) technology allows a User Equipment (UE) to determine whether PDCCH information sent by a Node B is lost or not. Specially, a DAI identifies PDCCH information of a next downlink sub-frame of a current bundling window. For example, 2 bits in a DAI field can indicate 4 values, easily identifying the PDCCH information when a size of a bundling window (M) is 1, 2, 3, or 4. However, when TDD configuration for uplink and downlink is 5, then M is 9. Accordingly, the 4 values of the DAI field need to be re-used. For example, when the number of a current PDCCH is a, its DAI value is obtained by modulo operation with 4, as shown in Equation (1).mod(a−1,4)+1  (1)
In an LTE-Advanced (LTE-A) system, Carrier Aggregation (CA) technology is used to support higher transmission rates, i.e., at least three Component Carriers (CC) are aggregated to provide a larger work bandwidth. Based on the CA, a Node B sends downlink data to a UE using the at least three CCs. Accordingly, the UE provides ACK/NACK feedback information for downlink data of at least three CCs. Herein, each CC is referred to as a “cell”.
According to current discussion for LTE-A, up to 4-bit ACK/NACK transmission is supported, based on channel selection technology. More specifically, a Frequency Division Duplex (FDD) system uses a 4-bit mapping table as shown in Table 1 below. In Table 1, ACK/NACK channels 1 and 2 correspond to two ACK/NACK bits of a Primary cell (Pcell), and ACK/NACK channels 3 and 4 correspond to two ACK/NACK bits of a Secondary cell (Scell). In Table 1, N means NACK, A means ACK, and D means Discontinuous Transmission (DTX). In Table 1, the feature is used that the two ACK/NACK channels of the same cell are always available or unavailable, thus the feedback performance and power controlling are optimized.
TABLE 1ACK/NACK channelPcellScell1234A, AA, A−1A, NA, A−jN, AA, A−jN, NA, A−1A, AA, N  jA, NA, N  1N, AA, N  1N, NA, N  jA, AN, A−1A, NN, A  jN, AN, A−jN, NN, A  1A, AN, N−1A, NN, N  jN, AN, N−jN, NN, N  1A, AD, D−1A, ND, D  jN, AD, D−jN, ND, D  1D, DA, A−1D, DA, N  jD, DN, A  1D, DN, NNo transmissionD, DD, DNo transmission
Another example of a 4-bit mapping table is shown in Table 2 below. In Table 2, a corresponding ACK/NACK channel is selected for transmission only when a certain piece of ACK/NACK information is an ACK. There is an exception, in that in order to fully use feedback capability of the M (M is 2, 3, or 4) channels, when a first piece of ACK/NACK information is a NACK and other pieces of ACK/NACK information are NACK or DTX, a Quadrature phase-shift keying (QPSK) constellation point of the first channel can function as an indication. Table 2 is used when the 4 ACK/NACK bits and their corresponding ACK/NACK channels are separated.
TABLE 2QPSKACK/NACK informationACK/NACKconstellationb0b1b2b3channelpointDN/DN/DN/DDTXNN/DN/DN/Dh0  1AN/DN/DN/Dh0−1N/DAN/DN/Dh1−jAAN/DN/Dh1  jN/DN/DAN/Dh2  1AN/DAN/Dh2  jN/DAAN/Dh2−jAAAN/Dh2−1N/DN/DN/DAh3  1AN/DN/DAh0−jN/DAN/DAh3  jAAN/DAh0  jN/DN/DAAh3−jAN/DAAh3−1N/DAAAh1  1AAAAh1−1
A method for ACK/NACK information feedback in an LTE-A TDD system based on channel selection is currently being discussed, which is based on the above-described DAI design of LTE. First, the ACK/NACK information corresponding to each PDCCH is sorted in ascending order according to the DAI. If there is an SPS service in the PDCCH, its ACK/NACK information is given priority. Thereafter, a number of consecutive ACKs starting from first ACK/NACK information is fed back to each cell.
More specifically, when the size of the bundling window M is 3, there are 4 possible values for the number of consecutive ACKs of each cell, that is, 0, 1, 2, or 3. Accordingly, these four values can be mapped to 2 bits, as the example shown in Table 3 below. When the size of the bundling window M is 4, there are 5 possible values for the number of consecutive ACKs of each cell, i.e., 0, 1, 2, 3, or 4. Here, these five values could be transferred to 4 states to be mapped to 2 bits. One of the transferring methods is shown in Table 4 below. In Table 4, N means NACK, A means ACK, D means DTX, and the symbol “/” means “or”.
TABLE 33 pieces of ACK/NACK information atmost in the bundling windowMapping statesA, A, AA, AA, A, N/DN/D, AA, N/D, anyA, N/DN, any, anyN, N/DD, any, anyD, N/D
TABLE 44 pieces of ACK/NACK information atmost in the bundling windowMapping statesA, A, A, N/DA, AA, A, N/D, anyN/D, AA, A, A, AA, N/DOr, A, D, D, DN, any, any, anyN, N/Dor, A, N/D, any, any except A, D, D, DD, any, any, anyD, N/D
According to the mapping states in Tables 3 and 4, 2-bit Pcell information and 2-bit Scell information are obtained, and ACK/NACK information could be fed back using the channel selection method according to the mapping of Table 1 or 2.
Allocating 4 candidate ACK/NACK channels for feeding back 4 bits information to support channel selection is also under discussion.
More specifically, for the ACK/NACK channel corresponding to the 2 bits of a Pcell, when there is SPS service, the first ACK/NACK channel is a semi-static configured ACK/NACK channel for SPS service, and the second ACK/NACK channel is determined by the PDCCH scheduling a Pcell with its DAI of 1. For example, if a minimum Control Channel Element (CCE) index of a PDCCH is n, one ACK/NACK channel could be mapped with the LTE method according to the CCE index n. When there is no SPS service, the two ACK/NACK channels are determined sequentially by PDCCHs of two Pcells with their DAIs of 1 and 2, respectively. For example, if the minimum CCE index of the PDCCH with its DAI value of 1 is n1 and the minimum CCE index of the PDCCH with its DAI value of 2 is n2, then the first ACK/NACK channel can be mapped with LTE method according to the CCE index n1, and one ACK/NACK channel can be mapped with LTE method according to the CCE index n2.
For the ACK/NACK channel corresponding to the 2 bits of an Scell, when cross-cell scheduling is used, the two ACK/NACK channels are sequentially determined by PDCCHs of two Pcells with their DAIs of 1 and 2, respectively. For example, if the minimum CCE index of the PDCCH with its DAI value of 1 is n1 and the minimum CCE index of the PDCCH with its DAI value of 2 is n2, then the first ACK/NACK channel can be mapped with the LTE method according to the CCE index n1, and the second ACK/NACK channel can be mapped with LTE method according to the CCE index n2. When no cross-cell scheduling is used, the two ACK/NACK channels are configured by a higher layer, and the ACK/NACK Resource Indicating information (ARI) in the PDCCH scheduling Scell is used to improve assigning flexibility.
According to the current LTE-A discussions, a power controlling command is still sent in a Transmit Power Control (TPC) field of the PDCCH scheduling Pcell, to ensure the Node B to control transmitting power of the UE. For the Scell using cross-carrier scheduling, a power controlling command is still sent in a TPC field of PDCCH scheduling Scell. For an Scell using non cross-carrier scheduling, a TPC field of PDCCH scheduling Scell is redefined as an ARI for indicating ACK/NACK channel resources, which means that the power controlling information cannot be carried in the TPC field. Accordingly, if the UE does not receive a PDCCH of a Pcell correctly, or the Node B does not currently schedule dynamic data in the Pcell, the UE will not receive the power controlling command, negatively affecting the reliability of ACK/NACK feedback information sent upstream.